Method of heat treating tungsten wire or ribbon



-12,1 6 E. c. HURST 3,165,427

METHOD OF HEAT TREATING TUNGSTEIN WIRE OR RIBBON Filed Aug. 24, 1962 INVENTOR. Edmond C Hurst excessive grain growth under. operating conditions.

3,165,427 Patented Jan. 12, 1965 United States Patent 'fiice This invention relates to methods of heat treating and more particularly to methods of heat treating pure tungsten wireqor ribbon.

Tungsten wireorribbonis widely used in the electrical art as filaments for electronic tubes, electron guns and incandescent lamps. Depending upon the desired use, the filament may be formed into many different shapes such as coils, helixes or straight strips. conventionally, the filament is heat. treated before use by flashing in a hydrogen atmosphere to develop and stabilize a crystal structure which will prevent warping or failure due to Since this heat treatment leaves the tungsten wire or ribbon brittle'and hencedifliculttoform, the filament is usually formed: into its desired shape prior to heat treatment.

The. characteristics of pure conventional tungsten filaments as a function of temperature were set forth by Jones and,Langmuir,in 1927. For example, a 10 mil tungsten wire filament operated at 2500" K. with a heater current of 6-.18amperes yields an emission of 60.3 ma. per inch. The life of a. tungsten filament is gen erally considered expended. when the diameter of the filamentihas beenreduced by 10 percent. due to evaporationsince at this-point the filament has a tendency to develop hot spots with resulting burnout of the filament. Thus, where a conventional filamentis operated at higher temperatures to obtain higher emission there.- from, its life is correspondingly reduced. 7

It is therefore onev object of the present. invention to provide a method for heat. treating tungsten wire or ribbon so that high emission may be obtained from the tungsten at low operating temperatures.

It is another object of the present invention to provide a method for heat treating tungsten wire or ribbon so that the tungsten is semiductile after the heat treating thereof.

It is another object. of the present invention. to provide a: method for heat treating: tungsten wire or. strip to obtain a high emission, semiductile, tungsten filament with nonsag characteristics.

It is another object of the present invention to provide a method of heat treating. tungstenxwire so that the wire may beformed into filaments after heat treating.

Other objects of the present invention will become more apparent as the detailed description proceeds.

In general, the method of the present invention comprises placing the tungsten wire. or ribbon in an evacuable chamber together with a metal member spatially mounted her negative. are maintained applied'to the tungsten and the member for a predetermined interval after which the A.-C. current is gradually decreased to zero amperes and the D.-C. voltage is removed. The polarities of the member and the tungsten are then reversed with the D.-C. voltage beng applied thereto. The A.-C. current is increased until rated heater current for the tungsten is flowing therethrough. The D.-C. voltage is then increased in val-- ue until maximum emission is obtained from" the tung-,

FIGURE 2 is a view along line 2 2 of. FIGURE 1 to illustrate the mounting of tungsten wire therein-.-v

FIGURE 3 is a view' of the mounting of tungsten ribbon for the apparatus of FIGURE 1. v

The apparatus of FIGURE 1 comprises a bell jar 10, a metal plate 12 and a seal 14-which seals the bell jar 10 to the plate 12. An aperture 16 in plate 12 gives access to a vacuum pumping system (not shown) which evacuates the bell jarv 12. Two clamps 18' and 20 of a suitable material such as molybdenum are mounted through the plate 12 and insulated therefrom by bushings 22. as molybdenum. is mounted on av support 26 between the clamps 18 and '20. The support 26 is mounted through the plate 12 and insulated therefromby a bushing 28. An A.-C. power supply (not shown) is connected between clamps 18 and 20- and a D;-C. power supply (also not shown) is connected between' one' of the clamps 18 and 20 and the support 26.

For the heat treatment of. pure tungsten wire according to the present invention, the tungsten wire30 is first passed through the tube 24- and clamped in' clamps'18 and 20. The wire 30 is thus mounted so that'it. lies along the longitudinal axis of the tube 24 and is in spatial relationship with the. walls thereof. For 10 and 20 mil tungsten wire a molybdenumtube of' /2. inch ID. was used, thewire being spaced approximately inch from the walls thereof. The bell jar 10 is then lowered'and sealed to the plate 12 by means of seal 14. The vacuum pumping system is started and the atmosphere within the bell jar 10 is evacuated to a pressure of at least 1 10- mm. of Hg.

An A.-C. current is applied to the wire 30 from'the A.-C. power supply. The amperage of the current is steadily increased. over a period of approximately 10-30 seconds to a value where the temperature of the tungsten wire 30 is just under the melting point thereof. The A.-C. current, and hence the temperature of the tungsten wire 30, is maintained at this value for a period of. approximately 3 minutes. Then, the DC. voltage of approximately 1000 volts is applied between. the tungsten wire 30 and the tube 24, the polarity of thetube" being negative and the polarity of the wire 30 being positive.

The 1000 volts D.-C. and the value of the applied A.-C. current are then held applied together for a period of approximately 5 minutes. The amperageof'the A.-C. current is then steadily reduced to zero amperesover a The A.-C. current and the D.-C. voltage I A metal tube 24 of a suitable material such period of 1-30 seconds. The D.-C. voltage is removed and the polarity of the tube 24 and wire 30 is reversed, the wire 30 becoming negative and the tube 24 becoming positive. The value of the A.-C. current is again increased to a value giving conventional rated heater current for the wire size under treatment (i.e., for 20 mil tungsten wire at 2500 K., 17.47 amperes). The value of the D.-C. voltage is then increased so that maximum emission is obtained from the wire 30 at the above described heater current. The A.C. current, D.-C. voltage, and partial vacuum are then removed to complete the heat treatment process.

The last steps of the process, namely the obtaining of maximum emission, may be eliminated where the particular tungsten wire is to be-placed in service and undergo the same treatment. Otherwise they are essential as they act as an aging process to the wire.

It is to be noted that the same process may be applied to tungsten ribbon except that as shown in FIGURE 3 the metal tube 24 of FIGURES l and 2 is replaced by flat metal plates'32 of a suitable material such as tungsten. It is preferred when heat treating tungsten ribbon to use two flat plates 32, however, the use of one flat plate has been found acceptable. Thus, for heat treating pure tungsten 5 x 100 mil ribbon, two tungsten plates 341, inch thick, each spaced from the ribbon A; inch were v used.

Using the above described process on 20 mil tungsten wire having a 3 inch length, an emission of 2500 milliamperes was obtained after heat treatment with 17.47

amperes A.-C. heater current at a temperature of 2500 peres heater current, this is approximately times the conventional emission as given by Langmuir and Jones. The surface finish and ductility for the ribbon was the same as for the 20 mil wire. When 10 mil pure tungsten wire was used, the emission again was high, but the duetility thereof was greatly increased to a degree that the wire could be tied in knots without breaking. Thus, complex forms of filaments could be manufactured therefrom quite easily subsequent to heat treatment.

As described, the process has been shown applicable to straight sections of pure tungsten wire or ribbon. However, it is equally applicable to preformed tungsten filaments.- As in the tungsten ribbon, when the process is applied to preformed filaments flat plates rather than the metal tube should be used. In all cas'esof heat treatment, the wire or ribbon after being heat treated was found to have excellent nonsag characterstics. Thus, 10 mil pure tungsten wire wound in a spiral filament having 12 inches of wire between supports and .005 inch spacing between coils did not visibly change when heated to 2300 C.

Persons skilled in the art willbf course adapt the teachings in the art to methods far different than those illustrated. Accordingly, the scope of the protection afforded the invention should not be limited to the methods shown in the drawings and described above but should be determined only in accordance with the appended claims.

What is claimed is:

1. A method for heat treating an elongated element of tungsten, comprising the steps of spatially mounting said element in an evacuable chamber, mounting a metal member adjacent said element in spatial relationship thereto, evacuating said chamber to a pressure of at least was f e cury, applying an A.-C. current to said element, increasing the value of said A.-C. current over a period of approximately 10 to 30 seconds until said element reaches a temperature just under the melting point thereof, maintaining the value of said A.-C. current for a period of approximately 3 minutes, applying a D.-C. voltage of approximately 1000 volts between said heated element and said metal member, said D.-C. voltage being applied so that the polarity of said element is positive and the polarity of said member is negative, maintaining the value of said A.-C. current and said applied D.-C. voltage together for a period of approximately 5 minutes, decreasing the value of said A.-C. current to zero ampere and removing said applied D.-C voltage.

2. A method for heat treatingan elongated element of tungsten, comprising the steps of spatially mounting said element in an evacuable chamber, mounting a metal mern ber adjacent said element in spatial relationship thereto,

evacuating said chamber to a pressure of at least 10- thereof, maintaining the value of said A.-C. current for a period of approximately 3 minutes, applying a D.-C. voltage of approximately 1000 volts between said heated element and said metal member, said D.-C. voltage being applied so that the polarity of said element is positive and the polarity of said member is negative, maintaining the value of said A.-C. current and said applied D.-C. voltage together for a period of approximately 5 minutes, decreasing the value of said A.-C. current to zero ampere, removing said applied DC. voltage, increasing the value of said A.-C. current to a .value giving conventional rated heater current for the element under treatment, applying a D.-C. voltage between said heated element and said metal member so that the polarity of said element is negative and the polarity of said member is positive, increasing the value of said D.-C. voltage to obtain maximum emission from said element at said rated heater current, and removing said A.-C. current. and said D.-C. voltage.

3. A method for heat treating tungsten wire, comprising the steps of spatially mounting said wire in an evacuable chamber, mounting a metal tube about said wire so that said wire passes along the longitudinal axis of said tube in spatial relationship to the walls thereof, evacuating said chamber to a pressure of at least 10 mm. of mercury, applying an A.-C.,current to said wire, increasing the value of said A.C. current over a period of approximately 10 to 30 seconds to heat said wire to a temperature just under the melting point thereof, maintaining the value of said A.-C.- current for a period of approximately 3 minutes, applying approximately 1000 volts D.-C. between said wire and said metal tube, said D.-C. voltage being applied so that the polarity of said tube is negative and the polarity of said wire is positive, maintaining the value of said D.-C. voltage and A.-C. current together for a period of approximately 5 minutes, decreasing the value of said A.-C. current to zero ampere, removing said D.-C. voltage, increasing the value of said A.-C. current to a value giving conventional rated heater current for said wire under treatment, applying a ,D.-C. voltage between said heated element and said tube so that the polarity of said wire is negative and the polarity of saidtube is positive, increasing the value of said D.-C. voltage to obtain maximum emission from said wire at said A.-C. heaterlcurrent value, and removing said A.-C. current and DC. voltage.

4. A method for heat treating tungsten ribbon, com-' prising the steps of spatially mounting said ribbon in an evacuable chamber, mounting'a metal plate adjacent said ribbon in spatial relationship thereto, evacuating said chamber to a pressure of at least 10* mm. of mercury, applying an A.-C. current to said ribbon, steadily increasing the value of said A.-C. current over a period of approximately 10 to 30 seconds to heat said ribbon to a temperature just under the melting point thereof, maintaining the value of said A.-C. current for a period of approximately 3 minutes, applying approximately 1000 volts D.-C. between said ribbon and said metal plate, said DC. voltage being applied so that the polarity of said plate is negative and the polarity of said ribbon is positive, holding the value of said D.-C. voltage and said A.-C. current together for a period of approximately 5 minutes,

steadily decreasing the value of said A.-C. current to zero ampere, removing said D.-C. voltage, applying a D.-C. voltage of'reverse polarity between said ribbon and said plate, said ribbon being negative and said plate being positive in polarity, increasing the value of said A.-C. current to a value giving conventional rated heater current for the ribbon size under treatment, increasing the value of said reverse polarity D.-C. voltage to obtain maximum emission from said ribbon at said heater current value, and removing said A.-C. current and reverse polarity D.-C. voltage.

References Cited by the Examiner UNITED STATES PATENTS DAVID L. RECK, Primary Examiner. 

1. A METHOD FOR HEAT TREATING AN ELONGATED ELEMENT OF TUNGSTEN, COMPRISING THE STEPS OF SPATIALLY MOUNTING SAID ELEMENT IN AN EVACUABLE CHAMBER, MOUNTING A METAL MEMBER ADJACENT SAID ELEMENT IN SPATIAL RELATIONSHIP THERETO, EVACUATING SAID CHAMBER TO A PRESSURE OF AT LEAST 10**-4 MM. OF MERCURY, APPLYING AN A.-C. CURRENT TO SAID ELEMENT, INCREASING THE VALUE OF SAID A.-C. CURRENT OVER A PERIOD OF APPROXIMATELY 10 TO 30 SECONDS UNTIL SAID ELEMENT REACHES A TEMPERATURE JUST UNDER THE MELTING POINT THEREOF, MAINTAINING THE VALUE OF SAID A.-C. CURRENT FOR A PERIOD OF APPROXIMATELY 3 MINUTES, APPLYING A D.-C. VOLTAGE OF APPROXIMATELY 1000 VOLTS BETWEEN SAID HEATED ELEMENT AND SAID METAL MEMBER, SAID D.-C. VOLTAGE BEING APPLIED SO THAT THE POLARITY OF SAID ELEMENT IS POSITIVE AND THE POLARITY OF SAID MEMBER IS NEGATIVE, MAINTINING THE VALUE OF SAID A.-C. CURRENT AND SAID APPLIED D.-C. VOLTAGE TOGETHER FOR A PERIOD OF APPROXIMATELY 5 MINUTES, DECREASING THE VALUE OF SAID A.-C. CURRENT TO ZERO AMPERE AND REMOVING SAID APPLIED C.-C. VOLTAGE. 